The invention relates to a method for presetting an ink feed in multi-color printing.
In order to increase the efficiency of a printing operation, measures are taken to reduce the set-up time of printing presses to a minimum. To save time and money different systems of a printing press are preset during the initial set-up phase. For performing a presetting, information concerning the print job to be processed is used, like for example the format and thickness of the sheets, and process information, like for example the inking system temperature and atmospheric humidity of the ambient air. In printing presses having zonally acting ink metering devices, like for example ink duct blades, and having ink metering rollers that act over the print width, like for example ductor rollers, the presetting is performed based on data related to zonal area coverage values in the image that is to be printed. In printing presses which use printing plates, these data are gathered from the optoelectronic scanning of the printing plate for the separate colors to be printed, or one uses image data, which are present during the generation of a printing image on a computer. According to the known correlation between the ink film thickness on a printed product and the density in the printed image for a given (primary) color, the gap between the ink metering elements and an ink duct roller that is assigned to it, and the color stripe width on a ductor roller can be preset. In systems having an automatic presetting, the aforementioned correlations between ink film thicknesses or density values and gap widths and ink stripe widths for the process colors cyan, magenta, yellow and black are stored for example in the form of characteristic curves in a control unit. In particular when printing packaging material often times special inks are used in addition to the regular process inks. Because the analysis of a multitude of characteristic curves for special inks is very complex, in practical applications the characteristic curves of the process inks are used. This method of presetting results in unsatisfactory results because, as a rule, special inks are to be printed with film thicknesses that differ from those of the process colors.
Published, Non-Prosecuted German Patent Application No. DE 44 01 536 A1 discloses the use of characteristic curves for controlling or regulating an operating process of a printing press. Based on characteristic curves of a virtual reference printing press and further in dependence of at least one further parameter, a numeric approximation method derives setting values for actuating elements in an actual printing press. This method relates to a procedure for printing with regular process colors, because deriving setting values from characteristic curves of a virtual reference printing press is not feasible due to the multitude of possible special inks.
It is accordingly an object of the invention to provide a method for presetting an ink feed in multi-color printing which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which reduces the set-up time of a printing press.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for presetting an ink feed in multi-color printing, the method includes the steps of:
deriving presetting values for zonal ink metering devices and for ink metering rollers acting over a printing width from a set value for a weight per unit area of a full tone area, zonal area coverage values and an ink stripe width; and
presetting, for each of a plurality of printing inks, the zonal ink metering devices and the ink metering rollers prior to printing, corresponding to an ink demand for an image to be printed.
According to the method of the invention, in order to derive the presetting values for the ink metering device and for the ink metering rollers the weight per unit area for a full tone area or solid ink region and the specific weight of the printing ink are used in addition to the area coverage values. That is why the method is particularly advantageous when printing with special inks or special colors, but it is not limited to special inks. Special inks are increasingly produced with computer controlled ink mixing systems and are used for printing packaging material. For determining the amount of printing ink that is needed for the application, the weight per unit area is determined as the amount of ink per printed full tone area or solid ink area. For standard ink mixtures, the weight per unit area is generally known within given limits of accuracy and can be used accordingly, so that in this case the printing of a test print for the determination of the weight per unit area can be avoided. The required ink film thickness can be calculated from the weight per unit area and the specific weight of the mixed-up printing ink. In printing presses with an ink gap that opens and closes between an ink metering element and an ink duct roller and with a ductor-type inking system, the ink zone opening to be preset is proportional to the calculated ink film thickness on a sheet or a web at a given lift or stroke of the ink duct roller.
For each of the inks or colors to be printed, in particular for each of the special inks or colors, a characteristic curve for the ink zone presetting can be created corresponding to the required ink film thickness.
The characteristic curves for the ink presetting can either be determined analytical, e.g. by using an ink unit simulation or can be determined empirical, by using print tests. One possibility of realizing the ink presetting for special inks is the use of correction factors on a characteristic curve of a base ink or process ink. The correction factors describe the film thickness ratios of a special ink to a base ink. Furthermore there is the possibility to use further parameters, like for example adhesive power and viscosity of the printing ink for the calculation of the presetting values. Aside from a reduced number of characteristic curves for the presetting, this approach has the advantage, that the characteristic curve can simply be adapted to modifications of the rheological characteristics of printing inks.
Another mode of the method according to the invention includes the step of deriving the presetting values by additionally taking into account a respective specific weight of given ones of the plurality of printing inks.
Yet another mode of the method according to the invention includes the step of determining the set value for the weight per unit area by using a test print wherein a spectral color measurement value of the test print corresponds to the set value for the weight per unit area.
A further mode of the method according to the invention includes the step of deriving presetting values for a special ink to be used for printing by using correction factors for presetting values provided for a base ink.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for presetting an ink feed in multi-color printing, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.